1. Field of the Invention
The present invention relates to a signal processing method using maximum likelihood decoding and a method for evaluating an information recording medium using maximum likelihood decoding.
2. Description of the Related Art
Recently, as the recording density for information recording mediums is being improved, the shortest mark length of recording marks is approaching the limit of the resolving power, which relies on a detection system.
In the case where, for example, an information recording medium is an optical disc medium, the “resolving power, which relies on a detection system” means an optical resolving power, which relies on the size of an optical spot generated by collecting laser light.
Due to the limit of the resolving power, increase of the inter-code interference and deterioration of SNR (Signal to Noise Ratio) are more conspicuous. As a result, a PRML (Partial Response Maximum Likelihood) method or the like is now used generally as a signal processing method.
The PRML method is a technology generated by combining partial response (PR) and maximum likelihood (ML). By the PRML method, with a premise that known inter-code interference occurs, a signal sequence having the maximum likelihood is selected from a reproduction waveform.
Owing to this, the PRML method is known to improve the decoding capability than a conventional level determination method (e.g., Non-Patent Document No. 1 (“Illustrated Blu-ray Disc Reader” (“Zukai Btu-ray Disc Dokuhon”) published by Ohmsha, Ltd.)).
In the meantime, because the signal processing method is now changed from the level determination method to the PRML method, problems occur in the evaluation method of a reproduction signal.
Jitter, which is a conventionally used reproduction signal evaluation index, is used with a premise that signal processing is performed by the level determination method. Therefore, occasionally, jitter may not be correlated with the decoding capability of the PRML method, which is based on a different signal processing algorithm from the level determination method.
Under the circumstances, new indices which are correlated with the decoding capability of the PRML method has been proposed (e.g., Patent Document No. 1 (Japanese Laid-Open Patent Publication No. 2003-141823) and Patent Document No. 2 (Japanese Laid-Open Patent Publication No. 2004-213862)).
Now, a case where the recording and reproduction quality is detected as distributions shown in FIG. 25 will be discussed.
FIG. 25 shows four distributions of differential metrics classified by the length of the space of 2T, 3T, 4T or 5T combined with a 3T-long mark, and the total of the four distributions. T represents a channel clock.
In FIG. 25, only 3T-long marks are classified as an example, but usually marks of other lengths are also classified.
In the case where classification is made by the length of the mark and the length of the space, there are the following two cases. In the case of FIG. 25(a), an SN component, which is the distribution width, is dominant in the distributions of in all the mark-space combinations. In the case of FIG. 25(b), an SN component of each pattern is good, but a shift component from the center of the distribution is different among the patterns. When the distributions are summed up, it appears that the SN component is poor.
The index described in Patent Document No. 1 (Japanese Laid-Open Patent Publication No. 2003-141823) cannot distinguish whether each distribution of differential metric is caused by an SN component or by a shift component.
Patent Document No. 3 (Japanese Laid-Open Patent Publication No. 2004-335079) solves this problem.
The index proposed in Patent Document No. 3 (Japanese Laid-Open Patent Publication No. 2004-335079) can detect a positional shift between a mark and a space (edge shift) by a combination of a mark length and a space length.
Namely, the level of the recording and reproduction quality obtained by the index proposed in Patent Document No. 1 (Japanese Laid-Open Patent Publication No. 2003-141823) can be distinguished as corresponding to an SN component or as corresponding to a shift component.
Owing to such distinguishing between an SN component and a shift component, it is now possible to analyze which type of error occurs in which pattern.
As described above, as the recording density of information recording mediums is more improved, the problems of the inter-code interference and SN deterioration will be more serious.
It is described in Non-Patent Document No. 1 (“Illustrated Blu-ray Disc Reader” (“Zukai Blu-ray Disc Dokuhon”), Ohmsha, Ltd.) that the system margin of an information recording and reproduction apparatus can be maintained by using a higher-order PRML method.
For example, when the recording capacity of one recording layer of a 12-cm optical disc medium is 25 GB, the system margin can be maintained by adopting the PR1221ML method.
The above-mentioned book describes that when the recording capacity of one recording layer is 33.3 GB, the PR12221ML method needs to be adopted.
Patent Document No. 3 (Japanese Laid-Open Patent Publication No. 2004-335079) proposes an index capable of detecting a positional shift of a combination of one mark and one space (edge shift). The positional shift represents the recording and reproduction quality of an information recording medium.
However, in an information recording medium having a higher recording density, there are marks and spaces which are much shorter than the length detectable by the resolving power of the detection system. Therefore, it is necessary to consider a positional shift including a plurality of edges provided by a combination of one or more marks and one or more spaces.
Hereinafter, a positional shift including a plurality of edges will be described.
The description will be given with a 12-cm optical disc medium for blue laser having a wavelength of 405 nm as an example.
According to Non-Patent Document No. 1 (“Illustrated Blu-ray Disc Reader”, Ohmsha, Ltd.), when blue laser is collected on an optical disc medium, the size of an optical spot is 390 nm. When the recording capacity of one recording layer using RLL(1, 7) as a recording code is 25 GB, the length of the shortest mark is 149 nm.
When the recording capacity of one recording layer of this optical disc medium is 33.3 GB, the length of the shortest mark is 112 nm. When the recording density is further improved, the length of the shortest mark is shorter.
Even where an identical detection system is used, when the recording density is 25 GB as shown in FIG. 26(a), the number of the shortest marks encompassed in an optical spot 201 is 2.6; whereas when the recording density is 33.3 GB as shown in FIG. 26(b), the number of the shortest marks encompassed in the optical spot 201 is 3.5. The length of the marks in the size of optical spot, which acts as the detection system for the optical disc medium, is shorter.
Therefore, an optical spot size may occasionally encompass a combination of a plurality of marks and spaces, instead of a combination of one mark and one space.
As a result, a signal influenced by a positional shift of a plurality of edges is detected, depending on the number of the marks and spaces encompassed in the optical spot size.
For example, a pattern in FIG. 27(a) in which one mark is sandwiched by two spaces, and a pattern in FIG. 27(b) in which one space is sandwiched by two marks, both include two edges. A pattern in FIG. 27(c), which include two marks and two spaces, includes three edges.
The index for evaluating the recording and reproduction quality of an information recording medium described in Patent Document No. 3 (Japanese Laid-Open Patent Publication No. 2004-335079) considers only the case where one edge shift of a combination of a mark length and a space length is included, and does not consider evaluating the recording and reproduction quality regarding a positional shift including a plurality of edges.
Non-Patent Document No. 1 (“Illustrated Blu-ray Disc Reader”, Ohmsha, Ltd.) describes that for a 12-cm optical disc medium for blue laser having a recording capacity per layer of 33.3 GB, the PR12221ML method needs to be adopted. Patent Document No. 3 (Japanese Laid-Open Patent Publication No. 2004-335079) describes that the proposed index is applicable to the PR12221ML method.
When the PR12221ML method is used for an optical disc medium using RLL(1, 7) as a recording code, the shortest marks are present in succession, and there is a pattern in which the square of the Euclidean distance between two ideal signals, i.e., a state transition path having the maximum likelihood and a state transition path having the second maximum likelihood, is 12.
The pattern in which the square of the Euclidean distance is 12 will be described later in detail.
The pattern in which the square of the Euclidean distance is 12 includes the shortest marks which are detected as a pattern including a plurality of edges as shown in FIG. 27.
It is now possible to provide an index representing a detection signal including information on a plurality of edge shifts which are detected by the PRML signal processing. However, provision of an index representing how each edge is shifted has not considered so far.